The most desirable form of test or predictive marker would be one which is indicative of some basic biochemical abnormality or defect in the hypertensive individual or in those individuals who would later develop hypertension, and which is likely to be related to the genetic predisposition of an individual to develop hypertension. Certainly no form of biochemical assay presently exists to detect those who may be predisposed to develop hypertension. Furthermore no biochemical test or assay has been developed which correlates unequivocally with all established hypertensive individuals. Existing biochemical assays appear to identify only certain subsets of hypertensives, i.e. only a certain proportion of hypertensive individuals display biochemical activities which are significantly different from the values determined for a comparable set of normotensive individuals (Garay, 1987). In this regard, the frequencies of Na.sup.+ transport abnormalities in the red blood cells of Caucasian hypertensive subjects and the resulting change in the concentration of Na.sup.+ in the red blood cell have revealed abnormalities in four major Na.sup.+ transport pathways. However, as stated above, the extent to which these individual pathways are abnormal in hypertension is certainly not uniform, nor is the ability of these Na.sup.+ transport abnormalities to discriminate between hypertensive and normotensive individuals, definitive. The maximal rate of red blood cell Na.sup.+ --Li.sup.+ countertransport activity has been reported to be increased in only 20% to 50% of subjects in a hypertensive study group while that of the (Na.sup.+ +K.sup.+) pump was increased in only 5% to 15% of the hypertensive group. An increased uptake of Na.sup.+ into the red blood cell via a passive "leak" pathway was reported to be present in only 10% to 30% of hypertensive individuals (Garay, 1987). The above are clear examples of the failure of existing tests to clearly and totally discriminate between hypertensive and normotensive individuals. Furthermore, they do not address identification of the pre-hypertensive individual.
Existing tests developed by others rely on the measurement of Na.sup.+,Li.sup.+ or H.sup.+ ion fluxes (whether singularly or in various combinations) across red blood cell, leucocyte or platelet cell membranes. Measurement of these ion fluxes have been done by direct means or indirectly by following cellular pH changes or swelling of a particular cell type. These tests have only been used in respect of adult subjects with established essential hypertension who have been compared with an appropriate control group of normotensive subjects who are matched for age, sex and body mass index. A complete list of these twenty or more studies are summarised in Table 6 and the full reference to each work is cited in the reference section. It must be pointed out that for some of these tests up to 120 ml of blood must be sampled in order to carry out such assays. Use of these tests in the case of pre-hypertensive subjects or with children has to our knowledge not been reported. The use of cheek cells in studies on existing hypertensives has also to our knowledge not been exploited by others.